The structure of conventional oil buffers for pressure with automatic reset is shown on the FIG. 1. It consists of a cylinder 101, a spring 102 and a piston 103 which is inside the cylinder 101. A piston rod 104 connects with the piston 103, a sleeve bearing 105 is mounted on the cylinder 101, seal rings 106 are between the piston rod 104 and sleeve bearing 105 and between the sleeve bearing 105 and cylinder 101. A case 107 is set on the cylinder 101, and a compressible bushing 108 is set between the cylinder 101 and the case 107. There are holes 109 for returning oil on the wall of the cylinder 101, a number of orifices 110 are arranged on the wall of the cylinder 101 along the axial direction. Holes 109 for returning oil and a check valve 111 are provided on the piston 103.
The operating principle is as follows: in case the piston rod 104 is pressed or impacted, the piston 103 compresses the spring 102 so that the spring 102 is compressed. The damping effect is realized by adjusting oil return speed from the right cavity to left cavity through the orifices 110 in the cylinder 101. The distances between two adjacent orifices are different. The speed will be changed gradually from high to low and to zero at the end. When an external force disappears, the piston 103 under the force from the string 102 pushes the piston rod 104 to move outwards. In this case, the check valve 111 on the piston 103 is opened, the oil in cylinder 101 may flow fast from the left cavity to the right cavity until the piston returns back to its original position.
There are following disadvantages for the conventional buffers: (1) it uses seal ring 106 between the piston rod 104 and sleeve bearing 105 to make sealing. When the piston rod 104 moving, the oil in cylinder 101 always sticks on the surface of the piston rod 104, after a period oil leakage will be happen. A vacuum will be finally appeared in the cylinder 101 because of this leakage after a period of operation. The oil leakage getting together causes the buffer finally be discarded as air is sucked into the cylinder 101. Thus, the service life of the buffer is short. (2) The space between the cylinder 101 and the case 107 are used to compensate the volume reduction of the cylinder 101 due to the piston rod entering into the cylinder 101. In order to ensure an effective travel of the piston rod 104, the diameter of the piston rod cannot be designed too large. Therefore, the rigidity of the piston rod 104 is low. It causes the buffer cannot endure a larger impact load.